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The Meridional Overturning Circulation The warm and salty water that spreads in the North Atlantic is cooled primarily by evaporation, sinks to the deep ocean and forms the North Atlantic Deep Water. The NADW flows southward along the Americas, and around the world, global upwelling occur, enters the SA at the intermediate water level South of Africa and return to the deep water formation regions through the SA.

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AMOC Changes at 16°N: The MOVE Array PI: Uwe Send 1 1 Scripps Institution of Oceanography, La Jolla, 1 Scripps Institution of Oceanography, La Jolla, California Continuous measurements of the lower limb of the AMOC at 16°N suggest a small decreasing trend since 2000. Such measurements, combined with extended time series results from RAPID-MOCHA, ARGO/altimetry, and developing programs at other latitudes, will determine the degree of “connectivity” of AMOC variability throughout the basin. MOVE array Southward limb of MOC Status :

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Science Requirements: Model-based observing system assessment Complete observing system evaluation for subpolar N. Atlantic and S. Atlantic Quantify impact of deep measurements in the interior North Atlantic (deep ARGO, full depth gliders) Develop observing systems capable of examining meridional coherence of the AMOC and associated elements, including forcing functions. Developing plans for putting judicious posterior error estimates on AMOC strength and variability estimates derived from ocean state estimation models. Develop coordinated activity (including model development and observing system sensitivity studies) to examine special physics region such as high- latitude sinking regions. Develop coordinated activity to examine water-mass transformation associated with AMOC and related air-sea interaction using modeling and assimilation systems. Understand the reasons for differences - or biases- in the relationship between model MOC intensity and MHT in available models, as compared to observations. Evaluate robustness of AMOC fingerprint in different models and consistency with observed fingerprint.

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Science Requirements: Climate Impacts Understanding the link between AMOC and SST variability : Study and better define the interaction between AMOC and upper ocean properties: SST, salinity, heat content. Understand the mechanisms of AMOC influence on SST and the possible role of upper ocean anomalies on generating (predictable) AMOC variability. There is a need to determine the phase relationships between the overturning circulation, the strength of the horizontal gyres, and the subsequent impact on SST. Robustness of AMOC/AMV impacts and connecting impact studies to societal needs : While the number of studies on AMOC/AMV impact on world (particularly Northern Hemisphere) climate has increased and broadened, more should be done to establish the robustness of these links, understanding their cause, and comparing them to other long-term changes in climate, forced and natural. It is important to link these impact studies to societal needs and if relevant to actual sectoral decisions (e.g., in water resources, coastal infrastructure, health, and ocean resource management). This will help focus impact research and lead to tangible benefits from AMOC research. Strengthen links to other related CLIVAR/WCRP activities : This will raise the profile of U.S. AMOC research and help in exchange of important information and stimulate progress. This goal can be achieved through U.S. CLIVAR, the International CLIVAR Atlantic Implementation Panel, and through communication with individual program committees, nationally and internationally.

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Solid black line: Moored instruments Black circle: planned Ocean Observatory. Blue lines: CTD lines. Dashed green lines: XBT lines, are repeated quarterly with the exception of the line GoodHope line (twice a year during the southern Hemispheric summer). Yellow lines: transit lines conducted every year and can be made available for Argo float deployments or any other observation. The third workshop for the South Atlantic Meridional Overturning Circulation (SAMOC 3) took place in Niteroi, Rio de Janeiro, Brazil on May 11-13, 2010. The Agenda, the list of participants, copies of the presentations and details on the observational plans can be obtained at: www.aoml.noaa.gov/phod/SAM OC/. Current plans for observations in 2010 and 2011 and recommended observations (Green rectangles).

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Numerical model studies completed in the United States, the United Kingdom, and in Brazil, as well as theoretical studies in the Netherlands, indicate that the higher latitudes (30  S to 34.5  S) are likely to produce more robust estimates of the MOC for several reasons. First and foremost, higher latitudes provide stronger density gradients, leading to improved signal-to-noise characteristics for geostrophic velocity calculations. Secondly, the strongest signals are more tightly confined to the boundaries at higher latitudes, particularly the eastern boundary, meaning the more intensely sampled array can be done in a smaller region. Thirdly, the measurement of the stability of the MOC, a crucial factor in attribution of observed signals, is more favorable at higher latitudes. The community recommendation is that a SAMOC monitoring array be located between 30  and 34.5  S, and that it involves 10-20 deep ocean moorings, a combination of tall moorings and pressure-equipped inverted echo sounders, coupled with several shorter direct velocity moorings on the shelf on either side of the basin. Furthermore, attribution of the observed signals at 30-34.5  S will require concurrent observations from Drake Passage and in the passage between Africa and Antarctica. Therefore, maintaining and augmenting the existing arrays in those areas will be crucial for success. Black is AMOC from OFES from V; Blue is reconstruction from a perfect T, S array, Red from a CPIES array (R. Perez)

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AMOC Program near term goals: 1. Observations Overturning in the Subpolar North Atlantic Program (OSNAP) Soft Lines: floats/gliders (US, ohers) Cape Farewell (UK) Reykjanes Ridge (FR) Ellett Line (UK) 53°N (Germany) Subpolar To quantify the large-scale, low-frequency, full water- column net fluxes of mass, heat and fresh water associated with the meridional overturning circulation in the subpolar North Atlantic.

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Green line: Site of past boundary monitoring: not presently in the water. The green line is the location of the UK DWBC and French EGC arrays, in the water between 2004 and 2008. Some engineering development is required to enable robust shelf monitoring here. Proposed seaward termination of this line is the planned OOI Irminger Sea Global-Scale Node (GSN) at 60°N, 39°W. Yellow lines: Ocean regions to be monitored by “soft” arrays, comprising of a mixture of gliders, floats, possibly deep T/S moorings, BPRs and bottom current meters. The observing design for the interior needs further study to assess the mix of instrumentation that can most efficiently accomplish the needed spatial and temporal coverage. O-SNAP Design elements: Soft Lines: floats/gliders (US, ohers) Cape Farewell (UK) Reykjanes Ridge (FR) Ellett Line (UK) 53°N (Germany) O-SNAP Red lines: Boundary monitoring partially implemented. The two red lines are (i) the German Labrador Sea Exit array, which would need to be extended both to near- surface and inshore on the shelf, and (ii) the Ellett Line, which needs improved temporal and spatial measurement density. Blue line: Approximate location of a possible Mid-Atlantic Ridge array.